Respectful

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late Middle English: from Latin respectus, from the verb respicere ‘look back at, regard’, from re- ‘back’ + specere ‘look at’.

wiktionary

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respect +‎  -ful

etymonline

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respectful (adj.)

1580s, "characterized by respect" (implied in respectfully), from respect (n.) + -ful. The meaning "full of outward civility" is attested by 1680s. Respectfully in conventional subscriptions of letters is attested by 1812. Related: Respectfulness.